Reagent-Controlled Synthesis of the Branched Trisaccharide Fragment of the Antibiotic Saccharomicin B

Article abstract of DOI:10.1021/acs.orglett.8b01355

A concise synthesis of a branched trisaccharide, α-l-Dig-(1 → 3)-[α-l-Eva-(1 → 4)]-β-d-Fuc, corresponding to saccharomicin B, has been developed via reagent-controlled α-selective glycosylations. Starting from the d-fucose acceptor, l-epi-vancosamine was selectively installed using 2,3-bis(2,3,4-trimethoxyphenyl)cyclopropene-1-thione/oxalyl bromide mediated dehydrative glycosylation. Following deprotection, l-digitoxose was installed using the AgPF₆/TTBP thioether-activation method to produce the trisaccharide as a single α-anomer. This highly functionalized trisaccharide can potentially serve as both a donor and an acceptor for the total synthesis of the antibiotic saccharomicin B.

Full text of DOI:10.1021/acs.orglett.8b01355

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Reagent-Controlled Synthesis of the Branched Trisaccharide
Fragment of the Antibiotic Saccharomicin B
Sameh E. Soliman and Clay S. Bennett*
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States
S
* Supporting Information
ABSTRACT: A concise synthesis of a branched trisaccharide, α-L-Dig-(1 → 3)-[α-L-Eva-(1 → 4)]-β-D-Fuc, corresponding to
saccharomicin B, has been developed via reagent-controlled α-selective glycosylations. Starting from the D-fucose acceptor, L-epi-
vancosamine was selectively installed using 2,3-bis(2,3,4-trimethoxyphenyl)cyclopropene-1-thione/oxalyl bromide mediated
dehydrative glycosylation. Following deprotection, L-digitoxose was installed using the AgPF6/TTBP thioether-activation method
to produce the trisaccharide as a single α-anomer. This highly functionalized trisaccharide can potentially serve as both a donor
and an acceptor for the total synthesis of the antibiotic saccharomicin B.
and enterococci.2 Given the pressing need for the development
accharomicin B, a heptadecasaccharide antibiotic, was first
of new classes of antimicrobials, saccharomicin B holds the
Sisolated and characterized in 1998 by Kong and co-workers
potential to serve as a next-generation antibiotic, especially if a
route could be developed that was flexible enough to provide
both the natural material and analogues. The biosynthesis of
saccharomicin B has yet to be fully elucidated,3 and the
promiscuity of the enzymes involved in the synthesis is
unknown. This leaves chemical synthesis as the only avenue for
the production of analogues. Before such analogue synthesis
will be possible, however, an efficient route to the parent
compound needs to be devised.
from the cultured broths of the rare actinomycete Saccharothrix
espanaensis (Figure 1).1 This class of antibiotics was found to
possess broad-spectrum activity, both in vitro and in vivo,
against a wide range of Gram-positive and Gram-negative
bacteria, including multidrug resistant strains of staphylococci
This daunting molecule possesses several structural elements
which would pose a major challenge to any attempted
synthesis. Saccharomicin B is composed of five different
deoxy-sugars including the rare L-4-epi-vancosamine (Eva), D-
saccharosamine (Sac), and L-digitoxose (Dig), which are
difficult to synthesize and install in a stereocontrolled fashion.
Historically, it has been extremely difficult to control the
stereochemical outcome of glycosylation reactions using deoxy-
sugar donors without recourse to lengthy sequences involving
temporary prosthetic groups or de novo synthesis,4 which
introduces several steps between each glycosylation reaction. As
a consequence, only two reports of studies directed at
fragments of saccharomicins have been reported to date, both
from McDonald’s lab.5,6 The first of these studies was directed
at the synthesis of the fucose−aglycone terminus of the
molecule. A later study was directed at the synthesis of both the
Figure 1. Structures of saccharomicin B, the target trisaccharide 1, and
the key deoxy-sugar monosaccharide building blocks (2−5).
Received: April 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01355
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Organic Letters
Letter
antipodal fucose−saccharosamine and fucose−saccharos-
amine−digitoxose structures.
primary amine, which was readily transformed into the
corresponding azide by the copper(II)-catalyzed diazotransfer
from freshly prepared trifluoromethanesulfonyl azide (TfN3)
solution in toluene.13,14 Subsequent benzylation under standard
conditions (BnBr, NaH, DMF, 0 °C) afforded the azide
derivative 10 (91% yield, over three steps). Finally, selective
removal of the anomeric TIPS group from 10 with tetra-n-
butylammonium fluoride (TBAF, 1 M in THF) in the presence
of acetic acid at room temperature cleanly provided the desired
L-4-epi-vancosamine hemiacetal 2 in 98% yield.
As part of a synthetic campaign directed at the total synthesis
of saccharomicin B, we sought to target the highly congested
trisaccharide 1 using our reagent-controlled α-selective
dehydrative glycosylation strategy.7 The challenges involved
in assembling the branched trisaccharide fragment 1 are, first,
the synthesis of a set of rare, suitably functionalized deoxy-sugar
building blocks 2−5, followed by the stereospecific con-
struction of the two α-glycosidic linkages.
Synthesis of the trisaccharide began with the construction of
L-4-epi-vancosamine. This is a rare, branched-chain 2,3,6-
trideoxy-aminosugar with an amine-bearing C-3 tertiary
center.8−10 Synthesis of the L-4-epi-vancosamine hemiacetal
building block 2 was effectively accomplished starting from
vancomycin-hydrochloride (Scheme 1). To this end, the amino
Synthesis of the 2,6-dideoxy-sugar, L-digitoxose,15 building
blocks 3 and 4 is outlined in Scheme 2. Zemplen de-O-
́
Scheme 2. Synthetic Pathway for L-Digitoxose Building
Blocks
Scheme 1. Synthetic Pathway for the L-4-epi-Vancosamine
Building Block
acetylation of the commercially available 3,4-di-O-acetyl-L-
rhamnal (11) using catalytic sodium methoxide, followed by
regioselective 3-O-silylation of the resulting diol with tert-
butyldimethylsilyl chloride (TBDMSCl) in the presence of
imidazole at 0 °C16 and subsequent etherification with 2-
naphthylmethyl bromide (NapBr) and sodium hydride in
anhydrous DMF at 0 °C, gave the 4-O-Nap ether protected
derivative 12 in excellent yield (92%, over three steps).
Desilylation (TBAF/THF) of the 3-O-TBDMS group, followed
by PDC/AcOH oxidation of the resulting allylic alcohol in ethyl
acetate at room temperature, gave the hex-1-en-3-ulose 13 in
73% yield over two steps. Low-temperature cesium carbonate-
promoted Michael-type addition of ethanthiol to the enone
derivative 13 in THF17 produced the thioethyl 2-deoxy-3-
uloside 14 in 86% yield as a single α-isomer. Sodium
borohydride (NaBH4) reduction of ketone 14 in a mixture of
MeOH−CH2Cl2 (3:1) at −15 °C was achieved with excellent
stereoselectivity. 1H and 13C NMR spectra of the crude product
showed a single product, containing an axial C3-OH,
corresponding to the desired L-digitoxose stereoisomer.
Subsequent benzylation of the crude alcohol furnished the
thioglycoside 3 in 94% yield over two steps. Chemoselective
hydrolysis of the anomeric phenyl thioether group with N-
bromosuccinimide (NBS) in wet acetone (10:1 acetone−
water)18,19 at 0 °C smoothly afforded the L-digitoxose
hemiacetal 4 in quantitative yield.
group of the vancosamine moiety on vancomycin-HCl was
subjected to Cbz carbamate protection, followed by acidic
methanolysis of the vancosaminyl sugar under anhydrous
conditions11 to give the corresponding methyl glycoside 6 in
87% yield.8a,12 Inversion of configuration at the C-4 atom of 6
was achieved via a stereospecific oxidation−reduction sequence,
as previously described by Kahne and Walsh.8b Thus,
pyridinium dichromate (PDC) mediated oxidation, in the
presence of acetic acid, afforded the 4-ketosugar 7 in 90% yield,
which upon subsequent reduction with sodium borohydride
(NaBH4) in a mixture of MeOH−CH2Cl2 (1:2) at −78 °C
furnished selectively the 4-epi-vancosamine derivative 8 in 97%
yield. Acid hydrolysis (4 M aq HCl, THF, 0 °C) of the methyl
glycoside 8, followed by regioselective O-silylation of the
anomeric hydroxyl group with triisopropylsilyl chloride
(TIPSCl) and imidazole in dry DMF, gave the silyl derivative
9 as a single β-anomer (85% over two steps). Catalytic
hydrogenolysis (Pd/C, H2) of the N-Cbz carbamate produced a
Synthesis of the D-fucose acceptors (Scheme 3) commenced
with the regioselective alkylation of the equatorial 3-hydroxyl
B
DOI: 10.1021/acs.orglett.8b01355
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Organic Letters
Letter
Scheme 3. Synthetic Pathway for the D-Fucose Building
Blocks
Table 1. Optimization of α-Stereoselective Dehydrative
a
Glycosylation for the Synthesis of Disaccharide 19
b
entry
temp (°C)
time (h)
yield (%)
1
2
3
4
5
rt
2
2
16
39
52
26
58
65
63
67
0
−10
−20
−10 → rt
−10 → rt
−10 → rt
−10 → rt
2
18
2
group of diol20 15, via dibutylstannylene acetal formation under
Dean−Stark conditions,21,22 using 2-naphthylmethyl bromide
(NapBr) to afford the 3-O-Nap ether building block 5 in 93%
yield. Benzylation (BnBr, NaH, DMF, 0 °C) of 5 at the 4-
position gave quantitatively the fully protected derivative 16,
and subsequent oxidative cleavage of the Nap ether with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in a mixture of
CH2Cl2−H2O (20:1) afforded 17 in 83% yield.
c
6
3
cd
,
7
8
3
ce
,
3
a
Acceptor (1 equiv), donor (2 equiv), thione (2.2 equiv), oxalyl
bromide (2.2 equiv), and TTBP (6 equiv) unless otherwise noted.
Yield of isolated product after purification. Reaction mixture was
stirred at −10 °C for 1 h before being allowed to warm to rt.
Acceptor (1 equiv), donor (3 equiv), thione (3.3 equiv), oxalyl
b
c
d
With the appropriate deoxy-sugar glycosyl donors and
acceptors in hand, we turned our attention to disaccharide
formation. To this end, 2,3-bis(2,3,4-trimethoxyphenyl)-cyclo-
propene-1-thione 18 was initially activated with oxalyl bromide
and then treated with the epi-vancosaminyl hemiacetal donor 2
and 2,4,6-tri-tert-butylpyrimidine (TTBP) in a mixture of 1,1,2-
trichloroethylene (TCE) and CH2Cl2, followed by the addition
of the Fuc-C4-acceptor 5 in THF at room temperature. This
dehydrative glycosylation condition afforded the desired
disaccharide 19 with complete α-stereoselectivity, albeit in a
modest 16% yield along with a substantial amount of trehalose
(Table 1, entry 1). After rapid screening of different reaction
temperatures and times (Table 1, entries 2−6), we were able to
achieve a satisfactory isolated yield (65%), with no impact on
selectivity, by performing all the additions at −10 °C and then
maintaining the reaction temperature at −10 °C for 1 h before
warming to room temperature (Table 1, entry 6). Increasing
the donor-to-acceptor ratio from 2:1 to 3:1 did not offer any
improvement to the yield (Table 1, entry 7). Interestingly, our
best result for the α-1,4-selective glycosylation of 2 with 5 was
obtained when the donor 2 was coactivated in the presence of
the acceptor 5 (Table 1, entry 8), affording disaccharide 19 in
67% yield. The α-(1 → 4) glycosidic linkage in 19 was
e
bromide (3.3 equiv), and TTBP (9 equiv). Donor was coactivated in
the presence of acceptor; THF was not used.
Scheme 4. Dehydrative Glycosylation of Disaccharide
Acceptor 20 with L-Digitoxose Hemiacetal Donor 4
1
confirmed by the H NMR spectrum, which showed a doublet
(δ 5.14 ppm) with a vicinal coupling constant J1,2 for H-1′ of
4.3 Hz.
Having established the conditions for the construction of
disaccharide 19, we turned our attention to the assembly of
trisaccharide 1. To this end, unmasking of the Nap protecting
group on 19 using DDQ afforded disaccharide 20 in 72% yield
(Scheme 4). With both the disaccharide acceptor 20 and the L-
digitoxose hemiacetal donor 4 in hand, we initially examined
the dehydrative glycosylation to directly forge the target
trisaccharide fragment. However, condensation of 20 with 4
under our cyclopropenium activation conditions proceeded in
poor yield, favoring the formation of the β-isomer. In hindsight,
this was not unexpected as digitoxose is known to preferentially
undergo β-selective glycosylation reactions.23
anomer (see Supporting Information). Our inability to utilize
our cyclopropenium method for the construction of 1 led us to
consider alternative approaches. To this end, we were attracted
to the mild activation of deoxy thioglycosides using silver
hexafluorophosphate (AgPF6) reported by Hirama et al.24a This
approach takes advantage of the different reactivity between 2-
oxy- and 2-deoxy-thioglycosides toward AgPF6. After evaluating
the scope of the reaction with the two reported24 non-
nucleophilic bases (DTBMP and TTBP) using D-fucose
thioglycoside 17 as a model C-3 acceptor (Table 2), we
found that the AgPF6/TTBP combination is the optimal
glycosylation promoter system for the stereoselective formation
of the L-Dig-α-(1 → 3)-D-Fuc linkage.
Attempts to optimize our cycopropenium cation-mediated
glycosylation on a model system failed to produce the desired
C
DOI: 10.1021/acs.orglett.8b01355
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Optimization of AgPF6-Mediated Glycosylation
Using D-Fucose Thioglycoside 17 as a Model C-3 Acceptor
ASSOCIATED CONTENT
■
a
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01355.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra for all new compounds (PDF)
b
entry
1
base
yield (%)
DTBMP
DTBMP
TTBP
62
44
72
AUTHOR INFORMATION
■
c
2
Corresponding Author
*E-mail: clay.bennett@tufts.edu.
ORCID
3
a
Acceptor (1 equiv), donor (1.5 equiv), AgPF6 (3 equiv based on
donor), and base (4 equiv based on donor) unless otherwise noted.
Yield of isolated product after purification. AgPF6 (3 equiv based on
b
c
Clay S. Bennett: 0000-0001-8070-4988
Notes
acceptor) and base (4 equiv based on acceptor).
Next, we focused on applying the optimized AgPF6
thioether-activation method to effect the final glycosylation to
assemble the target trisaccharide 1. To this end, a mixture of
disaccharide acceptor 20 and glycosyl donor 3 in the presence
of TTBP and 4 Å molecular sieves was treated with AgPF6 at
−10 °C for 3 h to give exclusively the desired trisaccharide 1 as
a single α-anomer in 76% yield (Scheme 5). Again, selectivity
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health (R01-GM115779)
for generous financial support.
REFERENCES
■
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Scheme 5. Synthesis of the Target Trisaccharide 1
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1
was confirmed by H NMR, which showed a doublet for the
anomeric proton of the digitoxose residue (δ 4.78 ppm) with a
vicinal coupling constant J1,2 of 4.7 Hz. In addition, 13C NMR
showed the anomeric carbon signals for the digitoxose and the
epi-vancosamine residues at δ 98.6 ppm (JC‑1,H‑1 168.7 Hz) and
δ 97.2 ppm (JC‑1,H‑1 173.0 Hz), respectively.
In conclusion, we have developed an efficient strategy for the
stereoselective synthesis of a branched trisaccharide 1, of
potential value for a future synthesis of the antibiotic
saccharomicin B, via reagent-controlled α-selective glycosyla-
tions of 2,6-dideoxy- and 2,3,6-trideoxy-sugars. The rare deoxy
sugar building blocks were readily constructed from commer-
cially available materials. Our cyclopropenium cation-mediated
reagent-controlled dehydrative glycosylation between L-4-epi-
vancosamine hemiacetal 2 and D-fucose C4-acceptor 5 gave 19
exclusively as the α-anomer. Following DDQ-mediated
oxidative cleavage of the 3-O-Nap ether, glycosylation of the
formed disaccharide acceptor 20 with the L-digitoxose
thioglycoside 3, promoted by AgPF6/TTBP, afforded the
desired α-linked trisaccharide 1 as a single diastereomer. This
highly functionalized trisaccharide can potentially serve as both
a donor and an acceptor for the total synthesis of saccharomicin
B, which is under investigation in our laboratory.
D
DOI: 10.1021/acs.orglett.8b01355
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Organic Letters
Letter
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E
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